Electric power translating apparatus for speed control of alternating current motors



March 28, 1961 ELECTRIC POWER TRA KAFKA ET AL NSLATING APPARATUS FORSPEED CONTROL OF ALTERNATING CURRENT MOTORS Original Filed April 5, 19558 Sheets-Sheet 1 2,977,518 us FOR SPEED T MOTORS 8 Sheets-Sheet 2 March1961 -w. KAFKA ETAL ELECTRIC POWER TRANSLATING APPARAT CONTROL OFALTERNATING CURREN Original Filed April 5, 1955 2,977,518 SPEED March28, 1961 w. KAFKA ETAL ELECTRIC POWER TRANSLATING APPARATUS FOR CONTROLOF ALTERNATING CURRENT MOTORS Filed April 5, 1955 8 Sheets-Sheet 3Original Fig. 8b

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ELECTRIC POWER TRANSLATING APPARATUS FOR SPEED CONTROL OF ALTERNATINGCURRENT MOTORS Original Filed April 5, 1955 8 Sheets-Sheet 7 March 28,1961 w. KAFKA ETAL ELECTRIC POWER TRANSLATING APPARATUS FOR SPEEDCONTROL OF ALTERNATING CURRENT MOTORS Origmal Filed April 5, 1955 8Sheets-Sheet 8 RST Fig 20 U i rd Swis Patent ELECTRIC POWER TRANSLATINGAPPARATUS FOR SPEED CONTROL OF ALTERNATING CUR- RENT MOTORS WilhelmKafka, Tennenlohe, near Erlangen, and Georg Sichling and ManfredTschermak, Erlangen, Germany, 'assignors to Siemens-Schu'ckertwerkeAktiengesellschaft, Berlin-Siemensstadt, Germany, a corporation ofGermany Original application Apr. 5, 1955, Ser. No. 499,416. Di-

vided and this application Oct. 11-, 1956, Ser. No. 615,301

. .Claims priority, application Germany Apr. "7, 1-954 6 Claims. (Cl.318-438) In applicants copending application, Serial No. 499,416, filedApril 5, 1955, now Patent No. 2,824,241, granted February 18, 1 958, andtitled Electric Power Translating Apparatus of Low Losses and LowWattless Power, there is described apparatus for translating alternatingcurrents and voltages for power distribution with minimum losses andminimum wattless power.

The present invention is a division of the above-mentioned applicationand is directed particularly to the translation of alternating voltageinto another alternating voltage of controllable frequency for operatingvariable-speed motors of the wound-rotor and synchronous types.

Translating operations of this kind, in general, are desired to incuronly moderate wattage losses and to avoid placing an appreciablewattless power demand upon the feeder line. This can be achievedwith theaid of dynamoelectric machines; but for many applications such rotatingmachines are undesired. Certain translating operations, in principle,are also possible with magnetic amplifiers or gaseous discharge devices,although magnetic amplifiers are applicable for currents and voltages ofonly one direction unless appreciable additional losses are to bepermitted. In any event, magnetic amplifiers and gaseous tubes when usedin alternating-current systerns for voltage or power regulation, such asby the delayed commutation method, impose a relatively large wattlessload upon the alternating-current feeder line.

It is, therefore, a more specific object of our invention toprovidepower translating apparatus for A.-C. motor control which is capable bymeans of static devices to translate power-current voltages of any kindand any wave shape at low wattage losses and with a more favorable powerfactor than heretofore attained with static translating means.

To this end, and in accordance with a feature ofour invention, weconnect into the circuit of the input voltage one or more rapidly actingswitching members and, by means of these members, chop the input voltageat the rhythm of a fixed or changing control frequency into individualvoltage sections; and we adjust the ratio of the make and breakintervals of the switching members so as to control the median values ofthe individual voltage section in accordance with the desired time curveof the output voltage. ireferably, we further provide filter means forsuppressing the upper harmonics resulting from the chopping operation.

According to a more specific feature of our invention, theabove-mentioned switching members consist of semiconductor devices ofcontrollable resistance and are controlled to abruptly vary theirresistance between minimum and maximum limits-in synchronismwith thecontrol frequency. v

According to still another feature of the invention, the voltagechoppingswitching members of the translating 2,977,518 Patented Mar. 23, 1961system consist of magnetically responsive semiconductor devices equippedwith a magnet structure in whose field the semiconductor member isdisposed and which has fieldwwindings for controlling the strength and/or polarity of that field.

Although in communication engineering the chopping principle has beenemployed for modulating purposes, for instance to minimize interferencein message transmission, the invention is based upon the recognitionthat, by virtue of the means herein disclosed, a chopping method is alsoapplicable for supplying electric power to motors in distributionsystems of utilities, with the advantage of not only reducing powerlosses, but also the wattless load imposed upon the feeder line.

The variation in ratio of make and break intervals, corresponding to themedian value of the motor-energizing voltage to be formed, can beeffected by varying the width (i.e. duration) of the voltage sectionsand/or by varying the chopping frequency. For instance, when chopping adirect voltage at a given constant frequency, the width of the resultingvoltage sections can be changed periodically from zero to a maximum andthereafter back to zero. This results in translating the input voltageinto median value corresponding to the half-wave of an alternatingcurrent. When thereafter the direct voltage is reversed in polarity andthe method is repeated, a second alternating-current half-wave ofreversed direction is obtained. It will be recognized, therefore, thatthe method is suitable for driving an alternating-current motor from asource of direct voltage.

If the chopping device used were ideal so that it had no interiorresistance thus completely interrupting its circuit within infinitesimaltime, then the chopping operati'on would not involve any wattage lossesat all. The subsequent formation of a median voltage value would requiresmoothing by reactive components, such as eapacitances or inductances,which involve a wattless load;

such interrupters are not applicable for attaining the ob jects of thepresent invention. Gaseous tubes are not applicable because they areincapable of interrupting the plate current other than by naturalextinction of the instantaneous current itself. In principle,high-vacuum tubes are applicable, but they are mechanically sensitiveand when conducting have a residual resistance so high as to causeseveral percent of voltage drop. In contrast, the above-mentionedsemiconductor devices are particularly well suitable for the choppingmethod according to the invention. They are controllable sufficientlyabruptly to change their resistance from maximum to minimum, and viceversa, the ratio between maximum and minimum resistance being very high.

Among the just-mentioned semiconductors and particularly suitable forthe purposes of the invention are the symmetrically conducting andmagnetically controllable semiconductors of high character mobility,that is semiconductors whose carrier mobility is about 600 cmF/volt sec.or more, and also asymmetrically conducting semiconductors of themagnetic barrier type.

The suitable semiconductors of high carrier mobility comprise those ofthe type A B that is compounds of any one of the elements boron,aluminum, gallium, indium in the third group (subgroup B) of theperiodic system with any one of the elements nitrogen, phosphorus,arsenic, antimony in the fifth periodic group (subgroup B).Semiconductors of this type, having greatly increased resistance whensubjected to a magnetic field, are described in the copendingapplications Serial No. 275,- 785, filed March 10, 1952, now Patent2,798,989; and Serial No. 391,647, filed November 12, 1953, bothassigned to the assignee of the present invention.

Semiconductors of the magnetic barrier type, of which intrinsicallyconducting and surface-treated germanium is preferably applicable, arelikewise distinct by a relatively low electric resistance which, byapplying a magnetic field of a given direction and of sufiicientstrength,

can be increased to a very high value in a given direction 1 of curentconductance. The high asymmetrical resistance of these semiconductorscan be wholly or partially eliminated by an additionally appliedelectric field or by radiation'of a given direction. semiconductingresistors exhibiting the magnetic barrier-layer effect are described inthe copending applications Serial No. 297,788, filed July 8, 1952, nowPatent 2,736,858; Serial No. 462,516, filed October 15, 1954; and SerialNo. 495,007, filed March 17, 1955,,all assigned to the assignee of thepresent invention.

Other types of controllable semiconducting resistors, for instancetransistors, preferably of the junction-type, and other very rapidlyoperating switching members of similar switching properties aresuitablefor the invention provided they are capable of chopping avoltage without appreciable losses at the required frequency.

The foregoing and more specific objects, advantages and features of theinvention will be apparent from and will be set forth in, the followingdescription with reference to the embodiments of the inventionillustrated on the drawing in which:

Figs. la to 1d show voltage characteristics explanatory of apparatus fortranslating high direct voltage into low direct voltage.

Fig. 2 shows schematically the circuit diagram of the simplest form ofsuch translating apparatus.

Fig. 2a shows schematically the circuit diagram of another suchtranslating apparatus, and Fig. 2b shows a modified portion of anotherwise similar apparatus.

Fig. 3 is a schematic voltage-time diagram exemplifying the translationof direct current into alternating current.

Figs. 4a and 4b show coordinate diagrams of voltage curves relating to atranslating operation for controlling the amplitude of an alternatingvoltage.

Figs. 5 and 6 are further voltage diagrams relating to a method forchanging the frequency of alternating voltage.

Fig. 7 shows the schematic circuit diagram of a systern for controllingthe speed of an induction motor by changing the frequency of analternating current supplied from a three-phase feeder line of fixedfrequency.

Figs. 8a, 8b and Figs. 9a, 9b represent further voltage curvesexplanatory of a system according to Fig. 7.

Fig. 10 shows the circuit diagram of a modified component for a systemotherwise similar to that of Fig. 7.

Fig. 11 is a voltage diagram explanatory of another modification in asystem according to Fig. 7.

Fig. 12 shows the circuit diagram of a modified component for use in asystem otherwise as shown in Fig. 7 and capable of performing theoperation according to Fig. 11.

Fig. 13 is the circuit diagram of another system for controlling thespeed of an induction motor by current of variable frequency suppliedfrom a three-phase line.

.Fig. 14 is a coordinate diagram representing speed versus slipfrequency and relates to the operation of a system according to Fig. 13.

Fig. 15a is the circuit diagram of a system for trans lating alternatingcurrent of constant frequency and con stant voltage into alternatingcurrent of variable frequency and variable voltage, Fig. 15b shows amodified detail and Fig. 15c an explanatory voltage and current diagram.

Fig. 16 shows explanatory voltage-time diagrams relating to theoperation of the system according'to Fig. 15.

Fig. 17 illustrates the schematic circuit diagram of a modifiedfrequency and voltage changer suitable for conductance.

' of a direct-current line supplying the voltage U Consider first thecase of converting a direct voltage with minimum losses into a directvoltage of lower magnitude. Figs. la to 1d exemplify two possibilitiesof such conversion. The feeder voltage U is periodically chopped intoindividual voltage sections of the duration T with intermediate pausesor break intervals T,,.

The chopping operation can be effected by means of an apparatusaccording to Figs. 2a or 2b. In Fig. 2a the load resistor R is connectedto the plus and minus buses A magnetically controllable semiconductormember S is connected in series with the load R and is controlled by awinding W so that its resistance is a minimum during the make interval Tand a maximum during the break interval T,,. Accordingly, during theinterval T the voltage U is impressed upon the load resistor, whileduring the interval T, the lower load voltage U is effective. Thesevoltage sections can be smoothed to a medium value U by means offiltering devices, for instance by the series connection of a dampingresistor R and a capacitor C connected parallel to the load resistor Ras shown. By varying the ratio of make interval T to break interval T,,,the median value of the converted voltage can be changed at willcontinuously between the limits U and U as will be recognized from Fig.la in comparison with Fig. 1b. However, one of the intervals, forinstance the make interval T may remain constant and the break intervalT may be varied so that the duration of the period T changes, as isrepresented by Figs. 1c and Id. In apparatus according to Fig. 2 thesmoothing components may be omitted in certain cases, particularly whenthe load R;, has inherent inertia, as is the case with incandescentlamps, furnaces, motors and the like.

In order to obtain the above-described control performance of Figs. 1aand 1b, the translating apparatus of magnetomotive force in the system Msource of alternating voltage of the desired control frequency. Thissource is represented by an alternator A The amplitude of the pulsatingcontrol voltage is adjusted or varied by means of a rheostat R The loadcircuit is shown to include in series a feedback winding W on it emagnctostructure M The feedback winding W coacts with windings W and Wto produce a flip-flop characteristic of the semiconductor resistance asis described in applicants above-mentioned copending application SerialNo. 499,416. The component magnetomotive force of winding W normallyproduces in semiconductor members a magnetic field perpendicular to thedirection of the load current axis so as to reduce the resistance to, aminimum. Only during one half-wave period of the pulsating coutrolcurrent in winding W can this winding act in diiferential relation tothe bias field winding W so that at a given phase point the magneticfield strength in member S declines below a critical value at which theresistance of the semiconductor is triggered up to its maximum value,thus causing the device to break the load circuit. At some timethereafter, the magnetomotive force of winding W again becomespreponderant so that the resultant field strength exceeds anothercritical value at which the resistance is triggered down to the minimumvalue thus making the load circuit. The phase moments at which the breakand make operations occur are set or controlled by means of therheostats R and R If the control winding is energized according to themodification shown in Fig. 2b, the apparatus is suitable for operationaccording to Figs. 1c and 1d. The control winding W in Fig. 2 is excitedby an RC tank circuit through a glow tube G1. By varying the setting ofthe control rheostat R the break period T can be varied, while the makeperiod T being determined by the ignition and extinction voltage of theglow tube G1, remains c011- stant. Otherwise the apparatus is as shownin Fig. 2a.

In this apparatus as well as in those described below, the switchingmember may be formed of a symmetrically conducting semiconductor of highcarrier mobility, preferably a crystalline body of a binary A B -typecompound such as indium arsenide or indium antimonide, both affording acarrier mobility above 20,000 cm. /volt second. However, in theapparatus of Fig. 2, as well as in those described below, the switchingmember may also be formed by a semiconductor of the magnetic barriertype, it being then necessary of course to observe the proper polaritiesof the voltages and the electromotive forces of the resistancecontrolling field windings of the semiconductor device. When thusproviding a semiconductor of the magnetic barrier type, itssemiconducting substance may have a lower carrier mobility and consistspreferably of germanium, although the above-mentioned indium compounds,as well as other semiconducting materials, including those previouslymentioned in this specification, are also applicable. For securing themagnetically controllable valve or rectifying effect, the semiconductormember is so prepared as to have a surface of high surface recombinationopposed to a surface or portion of low surface recombination. This isdone either by applying a surface treatment (see the above-mentionedcopending application Serial No. 297,788) and/or by giving thesemiconductor body a particular shape as disclosed in theabove-mentioned copending application Serial No. 495,007. For instance,relating to Fig. 2a, the following design is applicable.

One of the vertical surfaces of the semconductor crystal extendingparallel to the current-flow direction is given high surfacerecombination by grinding and polishing that surface to mirror finish.The other vertical surface parallel to the current flow is given lowsurface recombination by etching. This is done, for instance, byanodically treating the surface in an electrolytic bath. Suitable assuch a bath in the case of germanium is diluted caustic soda with someaddition of hydrogen peroxide.

In such a semiconductor member, the electron-hole pairs become crowdednear the surface of high recombination so that a carrier-depleted zone,i.e., the magnetic barrier layer, is formed in the zone of lowrecombination. This effect has the result that the semiconductor hashigh resistance to current flow in one direction and low resistance inthe other direction, depending upon the polarity of the applied magneticfield. Hence, the rectifying action can be reversed by reversing themagnetic field polarity. This barrier effect is increased by giving thehigh-recombination surface a larger area than the lowrecombinationsurface or reducing the latter surface to zero as described in theabove-mentioned application Serial No. 495,007; but this need not hereinbe further described because the particular design of the magneticiibarrier device is not essential to the present invention proper.

The semiconductor member is dimensioned in accordance with the requiredresistance and power values. For controlling high power values, it maybe necessary to subdivide the member into a number of individual crystalor/ and to provide them with cooling fins or other cooling means. Theindividual crystals are interconnected in series or parallel dependingupon the load requirements.

Another important application of the invention is for the conversion ofdirect current into alternating current. Such a conversion can beobtained in accordance with the method exemplified by Fig. 3. Duringeach halfwave to be formed, the feeder voltage is interrupted or choppedseveral times and the ratio of make and break intervals is so variedduring the half-wave period that the corresponding median voltage valueU corresponds to a sinusoidal half-wave, a reversal of polarity beingrequired after each half-wave.

Another important application of the invention is for the conversion ofalternating current into alternating current of different voltage ordifferent frequency. For instance, according to Figs. 4a and 4b, theeffective or median valued during each half-wave of an alternatingcurrent or alternating voltage is controlled by repetitively choppingthe current or voltage during each half-wave without changing the feederfrequency. Fig. 4a shows the adjustment of a low median value U of thevoltage derived from a feeder voltage U through the rapidly actingmembers. For comparison, Fig. 4b shows a control condition in which arelatively high median voltage value U is obtained. The two controlconditions differ in that the ratio of make to break intervals isrelatively small in Fig. 4a but relatively large in Fig. 4b. Thechopping frequency is the same in both cases. For obtaining the medianvalue U it is preferable to smoothen the upper harmonics of the highcontrol frequency at the load resistor. The circuit connection maycorrespond to Fig. 2., except that the apparatus is energized byalternating voltage instead of direct voltage. This fundamental andsimple circuit connection requires that the switching member S beconductive in both directions of current flow, a requirement readilysatisfiable with semiconducting resistance members. It will berecognized that a converting apparatus operating according to Figs. 4aand 4b functions much like a continuously regulatable transformer.

By reversing the polarity of the voltage sections during each secondhalf-wave, a direct current of controllable median value can beobtained. However, an alternating current can also be converted bychopping into an alternating current of a higher frequency which ismodulated by the alternating input current. This method is illustratedin Figs. 5 and 6. According to Fig. 5 the halfwaves ofthe input currentare chopped into individual sections, and these sections are alternatelyreversed in polarity relative to the load. The sine wave of the inputvoltage is shown at U The polarity-reversed sine wave is shown at U Inthis way a new, modulated alternating voltage of higher frequency isproduced, this voltage eing indicated by hatched areas.

Fig. 6 also exhibits how, additionaily, the voltage of the higherfrequency can be controlled by reducing the width of the individualvoltage sections relative to the values shown in Fig. 5, thus producinga correspondingly smaller median value for the individual sections. Ifthe width of the voltage sections remains constant but their timespacing is changed, then a change in frequency is obtained aside fromthe change in voltage magnitude. The polarity reversal may also beeffected after the elapse of several voltage sections. In all cases inwhich the input voltage is alternating, the translated voltage may besmoothed by resonance circuits tuned to the output frequency.

According to the present invention a translating operation as describedabove serves to produce alternating current of variable frequency and,if desired, also of variable voltage, particularly for producingmulti-phase current from direct current, alternating current orthreephase current, for the purpose of producing a rotating magneticfield of variable speed of rotation. With the aid of such a controllablerotating field, motors of simplest design can be used for controlling orregulating a drive. As a result, a machine without commutator becomessuitable for purposes which heretofore required the use of a Leonardconverter with two cornmutators or a three-phase commutator machine.This makes it possible to employ the most rugged type of electric motorfor operating under such unfavorable conditions as in a dust-ladenatmosphere or excessive heat, and to place'the appertaining controldevice at any desired remote location. Such systems provide athree-phase voltage of variable frequency, phase rotation and voltagemagnitude, and permit the operation of simple induction motors atcontrollable speed, with controllable brake performance and reversiblerunning direction. They can also be adjusted to a desired torque atstandstill. When providing the system with synchronous motors, severalsuch motors can be started and operated in exact synchronism, asis'required for certain follow-up control systems now being operatedwith rotating frequency-converters. For the regulation of inductionmotors and other rotating-field motors, the motor voltage must bechangeable from a minimum value at zero fre quency up to a maximum valueat the highest frequency in approximately linear proportion to thefrequency. The frequency can be controlled either independently of themotor or in dependence upon motor speed, motor torque, or any othervariable condition such as the speed of a second motor. That is, therevolving speed is then varied by controlling the resistance of theswitching members in response to the particular variable condition to beregulated.

The production of rotating fields of controllable speed and reversibledirection may be effected in various ways. Most readily understandableis an apparatus which includes an intermediate direct-current circuit orderives its energization from a direct-current feeder circuit.

The motor control system of Fig. 7 exemplifies a motorspeed controlsystem equipped with an intermediate direct-current circuit. The systemserves to operate a twophase induction motor M with a squirrel cagearmature from a 3-phase line RST at variable speed and reversible'rection. To this end, the three-phase current is at first convertedinto direct current of constant voltage by means of a rectifier 7, suchas a dry-rectifier set in threephase connection. If the inverter is tobe suitable also for the opposite power direction, then the rectifier 7must be of a type that permits being also operated as adirectto-alternating-current inverter, for instance a mercury arcrectifier wtih grid control. Each of the two stator Windings I and II ofthe motor M is connected in the diagonal branch of a bridge networkcomposed of the switching elements S to S or S to S and connected to therectified voltage. Connected in series with the switching members arerespective half-wave rectifiers 8 to 15, for instance, dry rectifiers.These rectifiers have the purpose of preventing the flow of current inthe inverse direction through the respective switching members withoutrequiring a reversing control of the switching members as such. For thatreason, the switching members may consist not only of magnetic barriermembers, but also of magnetically controllable semi-conductors of highcarrier mobility, and also of transistors. If the switching membersconsist of magnetic barrier semi-conductors or high-vacuum tubes, thenthe additional half-wave rectifiers 8 to may be omitted, provided theswitching members are controlled to reverse their forward direction atthe end of each half-Wave.

The operation of theapparatus will be explained with reference toFigs.8a, 8b and 9a, 912. These diagrams I with the extinction characteristicsL to L voltage in the phase windings I and II for a relatively highfundamental frequency and a high output voltage. Fig. 8b relates to thevoltage in windings I and II for low frequency and low voltage. As inthe case of Fig. 3, the direct voltage is converted into alternatingvoltage of the desired frequency by chopping, varying ratio of make andbreak intervals, and reversing the polarity of the voltage aftertermination of each half-wave. For controlling the switching members toproduce such curves, the control windings W to W for the phase I ofmotor M are connected in series, as shown in Fig. 7; and these windings,are controlled by a device 4 which supplies control pulses in the rhythmapparent from Fig. 8a. For this purpose, a small middle-frequencygenerator 16 ofany desired type may be, used for furnishing the choppingfrequency, a frequency changer 17 being connected in series with thegenerator. The frequency changer 17 supplies a voltage whose frequencycorresponds to the desired speed of the rotating field and whoseamplitude corresponds to the operating voltage to be impressed upon themotor M. Suitable as the frequency changer 17 is a conventional rotatingfrequency-converter machine with stator windings and a commutator withslip rings. This machine is shown driven by an auxiliary variable-speedmotor M The adjusted speed of the auxiliary motor lvl determines thespeed of the main motor M.

The control for the motor phase H has an analagous design and operation,except that a voltage of the same frequency as that of the frequencychanger 17 is interposed between the terminals 19 in series relation tothe middle frequency generator 13, this interposed voltage beingdisplaced one quarter period relative to the voltage of frequencychanger 17. The auxiliary generator 18 may be formed by a second windingon the generator 16.

The interposed voltage between the terminals 19 may be supplied from asecond pair of brushes on the frequency changer 17, as illustrated.

It may be added that the switching members are preferably given aflip-flop characteristic so that, as already mentioned with reference toFig. 2, the resistance is suddenly triggered from one to the other limitwhen the control magnitude passes through a given critical value. Sincethe sudden reduction in resistance is comparable to the ignition of agaseous discharge device, the value of the control magnitude at whichthe resistance jumps to its minimum is hereinafter called the ignitionvalue. The ignition values for different feeder voltages then form avoltage characteristic, hereinafter called ignition characteristic. Inanalogy, the characteristic indicating the values of control magnitudeat which the resistance is triggered to its maximum, is hereinaftercalled the ex tinction characteristic.

According to Fig. 9a, the make and break moments of the switchingmembers in Fig. 7 occur at the points where the full-line curve 27. ofthe control current intersects the broken-line ignition characteristicsZ to Z, of the respective switching members S to S and at theintersections That is, at these points of intersection the switchingmembers are triggered from low to high resistance of vice versa. Thecurve 22 of the control current results from the superposition of thelow-frequency voltage curve 23 shown by a broken line and the preferablytriangular voltage curve of the medium-frequency generator 6.Consequently a make pulse occurs at the intersection e and a break pulseat the intersection a The intermediate interval corresponds to the firstcurrent-conducting period (make period) according to Fig. 9b. Thesecond, somewhat longer make period in Fig. 9b is determined by theintersection points e and a in Fig. 9a, and so forth.

The duration of the make periods, therefore, increases up to the maximumof the low-frequency control voltage I 23 and thereafter decreases, sothat a median value 24,

shown by a broken line in Fig. 9b, will result. This voltage value 24resembles the low-frequency voltage curve 23 in Fig. 9a, but in contrastto the relatively slight power in the control circuit, relates to theload circuit which carries the large power required for operating thedrive motor M (Fig. 7). The ignition characteristics Z Z and Z Zpreferably have a larger spacing from each other than corresponds totwice the amplitude of the triangular modulating voltage of generator16. This results without further aid in having only one of the switchingmembers 8;, S or S S in conducting operation at a time.

The control for the motor phase II operates accordingly and supp ies a90 lagging voltage. Smoothing means are preferably provided forsuppressing upper harmonics. The shunt capacitors 2i and 21 in Fig. 7serve this purpose. The more rapidly the frequency changer 17 is driven,the smaller becomes the frequency taken off the commutator. At thesynchronous revolving speed, corresponding to the frequency of thefeeder line RST, the frequency changer 17 supplies direct current andthe motor M receives direct voltage. Under these conditions, thebroken-line curve 23 in Fig. 9 becomes a straight line located beneaththe ignition characteristics Z Z Then the individual make periods are ofequal duration and no current reversal can occur, so that practically,the motor phase I is energized by direct current. This applies brakingtorque until the motor is at standstill. A reversal of the runningdirection can be obtained by shifting one of the phases 180 or byreversing the control in one phase. Instead of a two-phase motor anormal threephase motor may be used, it being only necessary tocorrespondingly enlarge the translating apparatus and its controldevices in adaptation to the three-phase arrangement.

The described control system is also a typical example of a novel typeof power-current amplification in which a controlling voltage or currentof any wave shape is translated into a corresponding voltage or currentwithin a circuit of much higher power capacity without incurring withinthe amplifier 'any power .losses exceeding those of rotary convertersand without the necessity of providing large energy-storing devices forthe compensation of Wattless power. The energy storing means stilldesirable for minimizing the upper harmonics of the superimposedauxiliary frequency have a very much smaller size than those required,for instance, with magnetic amplifiers or gaseous discharge rectifiers.

In the system according to Fig. 7, four controllable semiconductormembers are required for each motor phase. In contrast, the modificationshown in Fig. 10 operates with only two switching members for each motorphase. To make this possible, each motor phase is subdivided into twoseparate windings, only the circuit connection for the motor phase Ibeing shown in Fig. 10 because the connection for the other motor phaseII is the same. As apparent, each of the two windings Ia and lb of themotor phase I can be traversed by current of only one given directiondetermined by the poling of the halfwave rectifiers 25 and 26.Consequently, the semiconductor members S and S must be controlledaccordingly by the control magnet windings W51, W The control of thewindings W and W can be efiected by alternating current in the samemanner as described with reference to the windings W and W in Fig. 7.

Connecting a power-current switching member of the semiconductor type inseries with a separate half-wave rectifier as shown in Figs. 7 and 10permits using a relatively simple control device for the switchingmember. However, such a series connection of the semiconductor memberwith another valve is not necessary, if the semiconductor member iscontrolled to remain completely blocked duringeach second half-waveperiod of the low frequency. I

While in the embodiments'last discussed the chopping frequency isconstant and the width of the voltage sec tions is variable, the widthof the voltage sections may be kept constant and instead the choppingfrequency be varied. This amounts to varying the pauses or breakintervals between the individual voltage sections as mentioned abovewith reference to Figs. 10 and Id. A particularly simple design of thecontrol device can be obtained, for instance, if a combined controlmethod is used which varies the width of the voltage sections as well asthe intermediate pauses, as is illustrated in Fig. 11. When applyingthis method by means of apparatus as shown in Fig. 7, then Fig. 11 takesthe place of the diagram for the phase circuit I in Fig. 8a, An exampleof control means for securing such a combined control is illustrated inFig. 12 with reference to the switching members S to S of Fig. 7.

In Fig. 12 the control windings for the switching members S to 8., aredenoted by W W and W W The control windings Wei, W and W 2 W areconnected in series with respective glow discharge tubes 29, 30 acrossrespective capacitors 31, 32. If desired, a resistor 27 or 28, as shown,may be connected in series with each pair of control windings. The twocapacitors 31 and 32 are connected through respective resistors 33, 34to respective direct-current sources 35, 36. The capacitors are furtherconnected in series with a common source of controlling alternatingvoltage 171 which corresponds to the frequency changer 17 in Fig. 7. Thevoltage of the direct-current sources 35, 36 may correspond to about theignition voltage of the glow tubes 29, 39. The capacitor 31 is chargedabove the ignition voltage when the voltage of the alternating currentsource 171 acts cumul tively to the voltage of the direct-current source35. When the glow tube 29 is ignited, the capacitor 31 dischargesthrough the control windings W W and the switching members S and S (Fig.7) are controlled down to minimum resistance. The duration of thecapacitor discharge depends upon how much charging current isreplenished through the resistor 33. This, in turn, depends upon themagnitude of the voltage of the alternating-current source 171. When thevoltage drops below the ignition voltage of the glow tube 29, then thecontrol current drops to zero and the resistance of the switchingmembers S S jumps up to the maximum value. The length of time elapsinstill the next ignition of the glow tube 29 depends upon the magnitude ofthe voltage at the alternating current source 171. Consequently, thevoltage simultaneously determines the duration of the pause and thewidth of the individual voltage sections. Since the voltage of thedirect-current source 36 is in series with that of source 171 but is inopposed polarity relation to direct-current source 35, the ignitionvoltage at glow tube 30 is not reached during the particular voltagehalf-wave of the alternating current source 171 just considered. Hencethe switching members S and 8,; remain blocked. During the nextfollowing half-wave, the other circuit branch with control windings Wand W comes into action, and the control windings W and W of thefirst-described circuit branch remain unexcited. It will be recognizedthat the control voltage for triggering the switching members iscomposed of the frequency-adjustable voltage from the dynamo-electricfrequency changer 171 and the approximately triangular discharge voltagefrom the capacitor and glow-tube combination, both component voltagesbeing superimposed upon each other. In this respect the system of Fig.12 resembles that described above with reference to Figs. 7 and 9a.

The system of Fig. 7 as described above comprises an auxiliary frequencychanger (17, 19) for supplying the control voltage of the convertingapparatus. The magnitude and frequency of the control voltage areadjusted by varying the revolving speed of the auxiliary frequencychanger with the aid of a variable-speed pilot motor M Instead of anauxiliary frequency changer, however, the control voltage and theresulting speed control of the main motor may also be effected inaccordance control and regulation of a three-phase induction motor M.lte'rn 37 in Fig. 13 represents a translating apparatus comprising therectifier 7, the switching members S to S and the auxiliary rectifiers 8to shown in Fig. 7. Only one phase of the motor control circuit is shownin Fig. 13, the two others being of analogous design, each beingprovided with a control voltage. The control voltage is formed by thevoltages of a three-phase tachometer machine 38 coupled with the motorM. These voltages are supplied to a dynamo-electric, rotating-fieldconverter 39 which increases the frequency of the tachometer 38 by aslip frequency i This slip frequency f is proportional to the revolvingspeed of the rotor in the motorgenerator converter 39. The rotorrevolving speed is determined by the speed of a small pilot motor 40connected through a transmission gearing 41 with the converter 39. Thespeed of motor 40 is determined by the selected adjustment of apotentiometer 42 from which, depending upon the adjustment, a positiveor negative direct voltage is tapped off. This direct voltage iscompared with the voltage of the tachometer 38 rectified by a rectifier43, and the difference voltage is impressed upon the armature of thepilot motor 40. Motor 40 is separately excited by the field winding 40'.

The operation of the system is as follows: At standstill of the mainmotor M and with potentiometer 42 in the illustrated mid-position, novoltage is applied to pilot motor 40. The voltage of tachometer 38 islikewise zero. Consequently the translating device 37 does not receivecontrol voltage and does not supply voltage to the main motor M. Forstarting the motor M, the tap of potentiometer 42 is displaced from themid-position. This supplies voltage to the pilot motor 40.Simultaneously the frequency converter 39 receives excitation through anauxiliary excitation winding 44. The frequency converter then suppliesat its sliprings a low frequency i depending upon the speed of pilotmotor 40 and the transmission ratio of gearing 41. The low frequency fis translated by apparatus 37 into a three-phase voltage of the samefrequency which causes the main motor M to start running. Withincreasing speed of motor M, the tachometer machine 33 supplies voltageto the converter 39, and the converter furnishes to the translatingdevice 37 a control frequency increased by the slip frequency f As aresult, the main motor M accelerates with a torque proportional to thepredetermined slip frequency f The motor acceleration continues untilthe output voltage of the rectifier 43 equals thevoltage pre-set atpotentiometer 42.

Then the pilot motor 40 stops, and the speed of the main e motor M at noload is determined by the setting of the potentiometer 42. Under load,when an amount of slip must occur for overcoming the load torque, thespeed of motor M is smaller by the amount of the load-responsive slipthan the no-load speed n to which the potentiometer 42 is set. Therelation between motor speed and slip frequency is schematically shownin Fig. 14.

By changing the setting of potentiometer 42, any desired values ofno-load speed n can beadjusted within the available range. If duringoperation of the motor M the no-load speed n decreases, a negative slipfrequency will occur. That is, under these conditions the motor runsabove synchronous speed and is subjected to braking. By displacing thepotentiometer tap in the opposite direction from the mid-position, themotor is controlled to acsmart exceed a given limit. This can beobtained by connecting a threshold resistor 45 parallel to the armature40. The device 45 may consist, for instance, of two anti-parallel dryrectifiers as shown, or of any other voltage-dependent resistors, or ofdirect-voltage sources connected in series with a half-wave rectifier.

If instead of an induction motor M a synchronous motor is used, then asystem largely similar to that of Fig. 13 may be employed for speedcontrol, except that the rotary converter 39 is to be replaced by arotary transformer and the pilot motor 40 is to be substituted by acomparing device, such as a weighing beam subjected to two mutuallyopposed torques of which one is proportion- 'al to the output voltage ofrectifier 43 and the other proportional to the voltage tapped off thepotentiometer 42. For starting the synchronous motor an auxiliaryfrequency of suitable voltage magnitude is preferably ima pressed uponthe control windings of the translating apparatus 37.

Another way according to the invention of translating three-phasecurrent of constant frequency and voltage into current of variablefrequency and variable voltage resides in the method of cutting thesinusoidal voltage curves of the three-phases of the supply line bymeans of switching members into voltage sections which, in accordancewith the instantaneous values of the sine wave, have differentmagnitudes and dilferent polarities. These voltage-time areas orintegrals are switched by the switching members onto the individualwindings of the motor or other power consuming equipment so that a newvoltage curve similar to a sinusoidal curve but of a different,preferably lower, frequency will result. The same voltage sections ofthe original frequency may also be applied to several circuits.

three-phase transformer, are connected to the three-phase supply lineRST in delta connection and in series with respective switching member SS S For minimizing the effect of upper harmonics on the feeder line,suitably dimensioned capacitors 49, and 51 may be connected in parallelto the individual transformer circuits, as illustrated. The left-handsecondary windings 46" to 48' are connected through respective switchingmembers 8;, S S to the motor phase winding I, and the right-handsecondary windings 46 to 48" through respective switching members S S Sto the motor phase winding II As in the embodiment of Fig. 7, each motorphase winding may be shunted by a capacitor 52 or 53.

The system of Fig. 15a is provided with another transformer whoseprimary winding is connected to the same phase leads R, S to which thetransformer 46 is connected. Transformer 70 has two secondary windings71 and 72. Secondary winding 71 energizes the control winding W ofswitching member 8, through a voltage-responsive resistor 73 bysinusoidal voltage represented in Fig. by wave a The other secondarywinding 72 is connected across the voltage-responsive resistor 73through an ohmic resistor 74. As a result,

the resistor 73 is impressed by a voltage 14 which is not sinusoidal butis flattened to an approximately trapezoidal wave shape. This voltage istaken from the winding 72 with such a polarity that it is opposinglydirected relative to the sinusoidal voltage 14 The sum of the twovoltages u and u acts upon the control winding W This sum voltage isshown in Fig. 150 as the difference between the voltage ag and thenegatively entered voIt-' age -u It will be recognized in Fig. 15c thatthe switching member S is opened at the moment 1 and is again closed atthe moment t This also applies to the negative half-waves of the linevoltage, provided the switching member S is of the magnetic barrier typeso that it reverses its blocking direction together with the polarityreversal of the magnetic field while the polarity of the applied voltageremains the same or, analogously, the blocking direction reversestogether With the applied voltage while the magnetic-field polarityremains the same.

The same type of control may also be applied to a transistor whoseemitter and collector alternately exchange their operations and in whichthe voltage n determines the potential of the transistor base contactrelative to the one control electrode operating as the emitter at atime. The modified circuit diagram illustrated in Fig. 15b shows aconnection suitable for such an operation of a junction-type transistor75. The voltage u is applied through a transformer 76 to the controlelectrodes so that the desired control effect is obtained when thevoltage across the terminals a and b changes its polarity. Two valves 79and 80 are provided for the prevention of disturbing currents in thecontrol circuit. In Figs. 15a and 15b the respective correspondingterminal points are denoted by a to d. That is, the components shown inFig. 151) between the terminals a to d are to be inserted into a systemaccording to Fig. 15a instead of the components S and W In Fig. 15a thecontrol of the switching members 8., to S is illustrated only for theswitching member S The control principle corresponds to that applied tothe switching member S except that the alternating control voltage isnot taken from the line but is supplied from the voltage effective atthe field winding I of motor M. Accordingly, the voltages L1 14 and M22have different frequency. The control or regulation of the motor speedis effected by varying the capacitance of the capacitor 52 connectedparallel to the motor winding 1.

Analogously designed and operative control means are applicable for thecontrol of the switching members S and S The individual control meansfor members S and 8;, correspond to those described above with reference to the switching member S, except that the control means for theswitching member S have their transformer 70 connected across the phaseleads S and T, and the corresponding transformer for the control ofswitching member is connected to the phase leads T and R. Similarly, thecontrol means for the switching members S to S are designed andoperative in the same manner as the control means for the switchingmember 8,, described in the foregoing, except that the transformer 71for the control of switching members S7, S S is connected to the winding11 of the motor M, while the corresponding transformer for switchingmembers S and S is connected to the motor winding I corresponding to theconnection shown for switching member S When changing the motor speed byvarying the capacitance setting of capacitor 52 the capacitor 53 mustlikewise be given a correspondingly different setting. The operation ofthe system Fig. 15a will be further explained with reference to Fig. 16.

Fig. 16 shows, one below the other, the sine waves of the three-phasevoltages RS, ST and TR which are displaced 120 electrical relative toeach other. By proper control of the switching member S with theabovedescribed means, the switching member opens its circuit andcommences to conduct current during each half-wave period at the makemoment t At the moment t the switching member closes its circuit so thata voltage section is cut out of each half-wave. By displacing the makeand break moments t and in mutually opposite sense, the part-timeconductance interval during each half-wave can be decreased andincreased. The same control occurs with the semiconductor switchingmembers S and S with the proper phase displacement of 120 electrical.Consequently, between the moments t and t the secondary windings 46 to48' and 46" to 48" produce a voltage as represented by each diagonallyhatched area. In the three phases RS, ST and TR, individual ones ofthese hatched voltage areas, i.e. voltage time integrals, can beselected and can be assigned to the motor phase I and to the motor phaseII so that they form a median voltage curve in accordance with thedesired frequency. For instance, in Fig. 16 the vertically hatchedsections of the three phases RS, ST and TR form for the phase I of themotor a new voltage wave whose fundamental frequency is shown by abroken line. The fundamental frequency for the motor phase II is formedby the horizontally hatched voltage sections of the three supply phasesRS, ST and TR. The base frequency for this phase II is the same as forthe phase I but is displaced relative thereto.

As apparent from the composition of the two motor phases 1 and II inFig. 16, the fundamental waves of the two phases in the example underconsideration do not have equal magnitudes because each individualhalf-wave of phase I comprises three voltage sections while the voltageof phase II is composed of only two sections. However, this is notunduly detrimental for the operation of a squirrel-cage motor in a drivesystem according to Fig. 15a because the torque of such a motordecreases relatively little when the individual phases are energizedsymmetrically. Furthermore, the control performance according to Fig. 16is presented only for explaining the principle of operation. By adifferent subdivision of the individual half-waves in the three phasesRS, ST and TR into several voltage sections and by a dilferent selectionof these sections, a symmetrical two-phase or multi-phase system canalso be obtained.

The invention, therefore, makes it possible to cut individual voltagesections out of the voltage supplied from a multi-phase line and tocompose these sections into new voltages, wave shapes and frequencies insingle or multi-phase systems, while using for this purpose completelystatic circuit components, namely controllable semi-conductor devices,and providing two or more seriesconnected switching members which areperiodically con trolled according to a predetermined program andindependently of each other.

The translating system illustrated in Fig. 17 permits realizing the sametranslating principle as explained with reference to Figs. 15a to 16.The switching members 5, to S operate to cut individual sections out ofthe phase voltages R, S and T. That is these switching members make andbreak their circuit several times during each half-wave period. Theswitching members S to 5,; on the one hand and S to S on the other handselect individual voltage sections and compose the selected sectionsinto new voltage curves for the motor phases I and II. Contrary to thesystem of Fig. 15a, the embodiment of Fig. 17 does not requireintermediate power transformers. The control devices for the switchingmembers in Fig. 17 may be similar to those shown in Fig. 15a anddescribed above.

A change in frequency can also be effected by repeatedly chopping theindividual half-waves of the alternating supply voltage and givingindividual voltage sections, for instance each following section,respectively different polarities so as to produce voltage curves asshown in Fig. 18a for the three phases RS, ST and TR of mutual phasedisplacement. This affords a particularly large variety of possibilitiesto compose new alternating voltage curves from the individual positiveand negative voltage sections. For instance, a fundamental frequency maybe composed in the manner shown in Fig. 18b for the phase I of asquirrel-cage motor, the new voltage curve being shown by a broken line.The new voltage, as illustrated, is composed of individual positive andnegative sections of the original phase voltbetween the two superimposedvoltages.

15 ages RS, ST and TR. For the motor phase II a fundamental frequency of90 displacement can be composed in a similar manner.

The system illustrated in Fig. 19 performs such a frequency changingoperation. The exemplified purpose of the system is again the operationof a two-phase squirrelcage motor from a three-phase feeder line RST.The secondary side of the system is the same as in Fig. 15a, the samereference being applied to corresponding components in both figures. Theprimary side of the system in Fig. 19 is similar to that described withreference 'to Fig. 4 in that a reversal in polarity of the voltage atthe transformer primaries 46 to 48 is provided for by a pushpullconnection, namely, by giving the primaries a midtap and energizing oneof the half portions of each primary through a second switching memberS, S' or S' It is further possible to reverse the resistance control ofthe switching members so rapidly and in such a timely sequence that thechopping of the individual half-waves of an alternating voltage resultsin a constant median value of the individual voltage pulses. As aresult, and as illustrated in Fig. 20, an alternating voltage of higherfrequency and approximately constant amplitude of its fundamental waveis obtained according to the broken line 57, provided care is taken tohave the make intervals decrease toward the middle of each half-waveperiod and increase during the second portion of that period whilereversing the polarity of the individual successive voltage impulses.The upper harmonics then occurring can be suppressed by filter means asdescribed previously.

An alternating voltage may further be translated into a direct voltageof approximately constant median value. To this end and as illustratedin Fig. 21, the individual half-Waves of the alternating voltage arechopped into voltage sections in the same manner as explained withreference to Fig. 20 without reversing the polarity of the successivesections within the same half-wave, provided all voltage sections ofeach alternate half-wave are reversed in polarity for instance by meansof a bridge network. The filter components for smoothing the outputvoltage are then considerably smaller than required for the conventionalrectification of an alternating feeder voltage of the same frequency.This is of advantage for such applications where the filter means forsmoothing the direct current resulting from conventional rectificationwould cause time-delays detrimental to the operation of the motor beingenergized. This is the case, for instance, with highly sensitiveregulating devices in which the measuring value to be responded to isavailable as an alternating voltage which must be rectified before beingapplied to the regulating circuits proper.

If the feeder current used for a translating operation according to Fig.20 is single-phase alternating, then the median value of the outputvoltage of higher frequency is relatively low in comparison with theamplitude of the original alternating voltage. However, the amplitudesof the new-frequency voltage are considerably larger when the samemethod is applied .to three-phase feeder current and if care is taken,by timely com-mutating from each phase to the next following phase, toutilize only the peak portions of the sine wave of thefundamental-frequency voltages. It is then relatively simpleto obtainfor the resulting median frequency a constant amplitude becauserelatively slight changes in the make periods are sufiicient. If, forinstance, with a method according to Fig. 22, another voltage of thesame median frequency is produced and if the two voltages are imposedupon each other, then the amplitude of the resultant output voltage canbe varied simply by varying the phase displacement By continuouslyvarying the phase voltage, this method can also be used for subjectingthe amplitudes of the sum voltage to modulation with a new frequency. Byrectifying the sum voltage and changing the polarity of rectification atthe voltage zero passages, a new low frequency is ob i6 tained which canbe regulated in a simple manner. A similar result is obtained by givingthe two superimposed voltages somewhat different frequencies. The sum isa beat-frequency voltage whose rectification also results in furnishinga new lower frequency. It is possible therefore, in this manner to varya lower output frequency within a wide range simply by phase displacinga high frequency. a

The above-described method for the conversion of alternating current maybe applied for equalizing load peaks in distribution systems with theaid of a flywheel generator. The flywheel is driven from a motor, suchas a wound rotor motor or a synchronous motor, and one of theabove-described frequency changers is interposed between the drivingmotor and the power distribution line of the frequency Whose load peaksare to be equalized. The frequency changer is capable of continuouslyconverting the frequency of the line to a somewhat higher and/orsomewhat lower value f and, if desired, to also vary the voltageamplitude accordingly. For equalizing a load peak in the distributionline, the frequency value f is lowered by gradually varying thefrequency ratio so that the flywheel runs at supersynchronous speed andsupplies power. In the load valleys the frequency f is increased so thatthe flywheel consumes power. This control of the frequency changer canbe effected in dependence upon the load of the line or upon the currentor power consumption of consuming equipment. Such a system isadvantageous, for instance, for the operation of reversible rollingmills, or mine hoists and other hoisting equipment which usuallyinvolves large load peaks. I

In control systems as described above, it is important for securing lowlosses that during the chopping operation the transition from minimum tomaximum current and vice versa occurs with a minimum of losses in theswitching member. To this end, the time interval during which the changein resistance occurs can be shortened. On the other hand, the samepurpose can be served by subjecting the current characteristic resultingfrom the resistance change to modification so that the current increaseduring make operation is delayed and the current decline during breakoperation is accelerated. Such means for reducing the transitory lossesare more fully disclosed in the copending application Serial No.491,983, filed March 3, 1955, and assigned to the assignee of thepresent invention.

While in the foregoing we have described semiconductor devices asvoltage-chopping components of translating apparatus according to theinvention, it is for some applications also possible to use high-vacuumtubes instead of the semiconductors, although such tubes are less ruggedand have a high residual resistance when conducting. For instance, suchtubes may be used as switching members S to S in such systems as shownin Fig. 7 and following. In that case, when applying high-vacuum tubesfor power translation, flip-flop phenomena are less significant and neednot be used. The control range of the tubes then takes the place of thetrigger characteristics Z and L in Fig. 9a. That is, the characteristicsZ and I. then represent the limits of the grid voltages for maximumconductance and blocking of the tubes, respectively. In Fig. 9b thefrontal steepness of the voltage sections, which is essential for theresulting losses, increases and hence becomes more favorable with asmaller control range of the tubes.

It will be apparent to those skilled in the art, upon a study of thisdisclosure, that our invention permits of various modifications andapplications other than those specifically set forth herein, withoutdeparting from the essential features of our invention and within thescope of the claims annexed hereto.

We claim:

1. A motor control system, comprising voltage supply means, arotating-field motor, static switching means having a semiconductormember of controllable resistance connecting said motor with saidvoltage supply means and having periodic resistance control means joinedwith said semiconductor member for triggering said resistance betweenminimum and maximum to periodically chop said supply voltage intoindividual voltage sections to impress alternating output voltage uponsaid motor, said control means having a controllable source ofvariable-frequency voltage for controlling the frequency of said outputvoltage to thereby control the speed of said motor, said source ofvariable-frequency voltage comprising a dynamoelectric frequency changerhaving a voltage variable in approximately linear proportion to itsfrequency, and said control means comprising another source ofapproximately triangular voltage corresponding to a desired choppingfrequency higher than that frequency of said frequency changer, andcircuit means connecting said frequency-changer voltage and saidtriangular voltage superimposed upon other to said static switchingmeans.

2. In a motor control system according to claim 1, said other source sfsubstantially triangular voltage comprising a capacitor, a chargingcircuit connected with said capacitor, a glow discharge tube connectingsaid capacitor with said switching device, and resistance meansconnected with said capacitor for controlling the capacitor chargingvoltage to thereby determine the make period and frequency of thechopping operation of said switching device.

3. A motor control system, comprising voltage supply means, analternating-current motor, a static switching device of controllableresistance having a semiconductor member connected to said supply meansin series with said motor and having a magnetic field in whose fieldsaid semiconductor member is disposed, said field member having amagnetizing winding for controlling the resistance of said semiconductormember, a source of periodic current connected to said field winding forcontrolling said device to chop said voltage into individual voltagesections, and adjustable frequency-control means connected with saidsource for varying the make and break intervals of said switching deviceto adjust the median values of said voltage sections in accordance withthe adjusted frequency, whereby the speed of said motor is controllableby varying the adjustment of said frequency control means.

4. A motor control system, comprising voltage supply means, asquirrel-cage motor having a field winding, a static switching devicehaving a semiconductor member of variable resistance connecting saidsupply means with said field winding, said device having periodicresistance 18 control means connected with said semiconductor member forcontrolling said resistance of said member to chop said supply voltageinto individual sections, and a controllable source ofvariable-frequency voltage for varying the frequency of said controlmeans to thereby control the speed of said motor.

5. A motor control system, comprising voltage supply means having twobuses, an alternating current motor having two terminals, a staticswitching device connecting said motor terminals to said buses, saiddevice having two pairs of symmetrically conducting semiconductormembers of controllable resistance, each pair having its two membersconnected between one of said respective terminals and said respectivetwo buses, two halfwave rectifiers seriesconnected with said respectivemembers of each pair in mutually opposed poling relative to said motor,said device having four magnetic field means .in Whose respective fieldssaid four semiconductor members are located, and periodic-current supplymeans connected to said field means for causing them to trigger theresistance of said semiconductor members between maximum and minimum toperiodically chop the supply voltage into individual voltage sectionsWhereby said motor terminals are impressed by alternating voltage, saidperiodic current supply means having frequency-adjusting means forvarying the frequency of said alternating voltage to thereby control themotor speed.

6. A motor control system, comprising voltage supply means, aplural-phase alternating-current motor having a field winding for eachof its respective phases, a plurality of static switching devices forsaid respective phases, each of said switching devices connecting one ofsaid respective windings to said supply means and having a semiconductormember of controllable resistance and periodic resistance control meansfor triggering said resistance between minimum and maximum toperiodically chop said supply voltage into individual voltage sectionsto impress alternating voltage upon said respective windings, saidcontrol means having a frequencycontrollable source of mutuallyphase-displaced voltages connected with said respective switchingdevices, whereby the speed of said motor is controlled by change infrequency of said source.

References Cited in the file of this patent UNITED STATES PATENTS1,408,758 Meyer Mar. 7, 1922 2,719,944 Brailsford Oct. 4, 1955 2,784,365Fenemore et al. Mar. 5, 1957

